1. Field of the Invention
This invention relates to a chopper formed in a plastic sheet for use primarily in forward looking infrared (FLIR) systems and principally, but not limited to such systems, of the type utilizing uncooled ferroelectric infrared pyroelectric detectors.
2. Brief Description of the Prior Art
Forward looking infrared (FLIR) systems generally utilize a detector and a chopper system in conjunction with the detector for calibration of the detector. Such calibration is generally performed on-line and between detector scanning operations. Prior art infrared detectors have generally been of the cooled variety, operating at temperatures in the vicinity of liquid nitrogen, about 77.degree. K More recently, FLIR systems have been developed which use uncooled detectors, such systems being preferred when sufficient sensitivity can be obtained therefrom. An uncooled detector system utilizing a ferroelectric detector is intrinsically a differencing detector whose signal is the difference between that of the viewed scene and that of a reference source. In order to minimize dynamic range problems in the detectors, it is desirable to match the reference flux as closely as possible to the average scene flux. This is typically accomplished with the chopper which alternately permits the detector to view the scene and then view a reference source representing the average scene flux.
For purposes of minimizing the scene flux/reference flux delta, some FLIR systems have used as a reference source an image of the system exit pupil or an approximation of the system exit pupil. The most simple technique to approximate the exit pupil is to defocus the optical system. In the present day systems, this is accomplished in one of two ways, these being either (1) with a thick flat plate which is cut out in appropriate areas to pass the scene radiation, whereby, in solid areas, an optical defocus occurs, resulting in a pupil approximation or (2) with a solid flat plate which is covered with small ground lenslets in a pattern matching the solid area of a scanner, these lenslets accomplishing the defocus.
A problem with prior art choppers of the second type described above has been cost. In order to provide a chopper of the above described second type having a plurality of lenslets, it has been necessary to grind the lenslets individually, generally in germanium, to provide a predetermined pattern. Such prior art choppers have also been fabricated using binary diffractive optic pattern generated photomasks in conjunction with a high precision laser writer followed by etching of the desired lens patterns into the germanium wafer. Such processes have been costly. It is therefore desired to provide choppers at greatly reduced cost, preferably at a small fraction of the present cost.